Night monkeys, also known as owl monkeys or douroucoulis (), are nocturnal New World monkeys of the genus Aotus, the only living member of the family Aotidae (). The genus comprises eleven species which are found across Panama and much of South America in primary and secondary forests, tropical rainforests and cloud forests up to . Night monkeys have large eyes which improve their vision at night, while their ears are mostly hidden, giving them their name Aotus, meaning "earless".
Night monkeys are the only truly nocturnal monkeys, although some cathemeral populations of Azara's night monkey have irregular bursts of activity during day and night. They have a varied repertoire of vocalisations and live in small family groups of a mated pair and their immature offspring. Night monkeys have monochromatic vision which improves their ability to detect visual cues at night.
Night monkeys are threatened by habitat loss, the pet trade, hunting for bushmeat, and by biomedical research. They constitute one of the few monkey species affected by the often deadly human malaria protozoan Plasmodium falciparum and are therefore used as experimental subjects in malaria research. The Peruvian night monkey is classified by the International Union for Conservation of Nature (IUCN) as an Endangered species, while four are Vulnerable species, four are Least-concern species, and two are data deficient.
Taxonomy
Until 1983, all night monkeys were placed into only one (A. lemurinus) or two species (A. lemurinus and A. azarae). Chromosome variability showed that there was more than one species in the genus and Hershkovitz (1983) used morphological and karyological evidence to propose nine species, one of which is now recognised as a junior synonym. As is the case with some other splits in this genus, an essential part of the argument for recognizing this new species was differences in the chromosomes. Two extinct species of Aotidae are known from the fossil record: Aotus dindensis and Tremacebus harringtoni.
Classification
thumb|Three-striped night monkey
Family Aotidae
- Genus Aotus
- Aotus lemurinus (gray-necked) group:
- Gray-bellied night monkey, Aotus lemurinus
- Panamanian night monkey, Aotus zonalis
- Gray-handed night monkey, Aotus griseimembra
- Hernández-Camacho's night monkey, Aotus jorgehernandezi
- Brumback's night monkey, Aotus brumbacki
- Three-striped night monkey, Aotus trivirgatus
- Spix's night monkey, Aotus vociferans
- Aotus azarae (red-necked) group:
- Azara's night monkey, Aotus azarae
- Peruvian night monkey, Aotus miconax
- Nancy Ma's night monkey, Aotus nancymaae
- Black-headed night monkey, Aotus nigriceps
- Group Incertae sedis
- Aotus dindensis
- Genus Tremacebus
- Tremacebus harringtoni
Physical characteristics
Night monkeys have large brown eyes; the size improves their nocturnal vision increasing their ability to be active at night. They are sometimes said to lack a tapetum lucidum, the reflective layer behind the retina possessed by many nocturnal animals. Other sources say they have a tapetum lucidum composed of collagen fibrils. At any rate, night monkeys lack the tapetum lucidum composed of riboflavin crystals possessed by lemurs and other strepsirrhines, Night monkeys can live in forests undisturbed by humans (primary forest) as well as in forests that are recovering from human logging efforts (secondary forest).
Distribution
A primary distinction between red-necked and gray-necked night monkeys is spatial distribution. Gray-necked night monkeys (Aotus lemurinus group) are found north of the Amazon River, while the red-necked group (Aotus azare group) are localized south of the Amazon River. Red-necked night monkeys are found throughout various regions of the Amazon rainforest of South America, with some variation occurring between the four species. Nancy Ma's night monkey occurs in both flooded and unflooded tropical rainforest regions of Peru, preferring moist swamp and mountainous areas. This species has been observed nesting in regions of the Andes and has recently been introduced to Colombia, likely as a result of post-research release into the community. The black-headed night monkey is also found mainly in the Peruvian Amazon (central and upper Amazon), however its range extends throughout Brazil and Bolivia to the base of the Andes mountain chain. Night monkeys such like the black-headed night monkey, generally inhabit cloud forests; areas with consistent presence of low clouds with a high mist and moisture content which allows for lush and rich vegetation to grow year round, providing excellent food and lodging sources. The Peruvian night monkey, like Nancy Ma's night monkey, is endemic to the Peruvian Andes however it is found at a higher elevation, approximately above sea level and therefore exploits different niches of this habitat. however unlike the other red-necked night monkey species, it is not endemic to Brazil.
Sleep sites
During the daylight hours, night monkeys rest in shaded tree areas. These species have been observed exploiting four different types of tree nests, monkeys will rest in; holes formed in the trunks of trees, in concave sections of branches surrounded by creepers and epiphytes, in dense areas of epiphyte, climber and vine growth and in areas of dense foliage. These sleeping sites provide protection from environmental stressors such as heavy rain, sunlight and heat. Sleeping sites are therefore carefully chosen based upon tree age, density of trees, availability of space for the group, ability of site to provide protection, ease of access to the site and availability of site with respect to daily routines. Ranges between night monkey species often do overlap and result in interspecific aggressions such as vocalizing and chasing which may last up to an hour. In general, the technique used by night monkeys in insect capturing is to use the palm of the hand to flatten a prey insect against a tree branch and then proceed to consume the carcass. The gestation period for night monkey is approximately 117– 159 days but varies from species to species. Birthing season extends from September to March and is species-dependent, with one offspring being produced per year; however, in studies conducted in captivity, twins were observed. Unusual among the New World monkeys, they are monochromats, that is, they have no colour vision, presumably because it is of no advantage given their nocturnal habits. They have a better spatial resolution at low light levels than other primates, which contributes to their ability to capture insects and move at night. Night monkeys live in family groups consisting of a mated pair and their immature offspring. Family groups defend territories by vocal calls and scent marking.
The night monkey is socially monogamous, and all night monkeys form pair bonds. Only one infant is born each year. The male is the primary caregiver, and the mother carries the infant for only the first week or so of its life. This is believed to have developed because it increases the survival of the infant and reduces the metabolic costs on the female. Adults will occasionally be evicted from the group by same-sex individuals, either kin or outsiders.
Nocturnality
The family Aotidae is the only family of nocturnal species within the suborder Anthropoidea. Whereas other divisions of primates, including Strepsirrhini and Tarsiidae, include many nocturnal and cathemeral species, the anthropoids possess very few nocturnal species and therefore it is highly likely that the ancestors of the family Aotidae did not exhibit nocturnality and were rather diurnal species. The presence of nocturnal behavior in Aotidae therefore exemplifies a derived trait; an evolutionary adaptation that conferred greater fitness advantages onto the night monkey. Their responses to olfactory stimulus are intermediate between those of the prosimians and diurnal primate species, however the ability to use auditory cues remains more similar to diurnal primate species than to nocturnal primate species. To begin, resting during the day allows for decreased interaction with diurnal predators. Members of the family Aotidae apply predation avoidance in choosing strategic covered nest sites in trees. These primates carefully choose areas with sufficient foliage and vines to provide cover from the sun and camouflage from predators, but which simultaneously allow for visibility of ground predators and permit effective routes of escape should a predator approach too quickly. While diurnal species are stimulated by the appearance of the sun, in nocturnal species, activity is highly impacted by the degree of moon light available. The presence of a new moon has correlated with inhibition of activity in night monkeys who exhibit lower levels of activity with decreasing levels of moon light. These species exhibit mate guarding, a practice in which the male individual will protect the female he is bonded to and prevent other conspecifics from attempting to mate with her. Mate guarding likely evolved as a means of reducing energy expenditure when mating. As night monkey territories generally have some edge overlap, there can be a large number of individuals coexisting in one area which may make it difficult for a male to defend many females at once due to high levels of interspecific competition for mates. Night monkeys form bonded pairs and the energy expenditure of protecting a mate is reduced. Males will therefore also have to distribute themselves to be within proximity to females, this form of food distribution lends itself to social monogamy as finding females may become difficult if males have to constantly search for females which may be widely distributed depending on food availability that year. Generally, food sharing is not observed in nature as the search for food requires a great degree of energy expenditure, but in the case of night monkey males, food sharing confers offspring survival advantages. As lactating females may be too weak to forage themselves, they may lose the ability to nurse their child, food sharing therefore ensures that offspring will be well feed. It is therefore likely that increased olfaction capacities improved the fitness of these nocturnal primate species; they produced more offspring and passed on these survival enhancing traits. There is a preference for scents of a particular type; those which indicate reproductive receptivity, which increases species fitness by facilitating the production of offspring. In the case of A. miconax, coffee plantations with introduced shade trees, provided quality habitat spaces. While the coffee plantation benefited from the increased shade—reducing weed growth and desiccation, night monkeys used the space as a habitat, a connection corridor or stepping stone area between habitats that provided a rich food source. This trait caused them to be recommended by the World Health Organization as test subjects in the development of malaria vaccines. Up to 2008, more than 76 night monkeys died as a result of vaccine testing; some died from malaria, while others perished due to medical complications from the testing.
Increased research and knowledge of night monkey ecology is an invaluable tool in determining conservation strategies for these species and raising awareness for consequences of the anthropogenic threats facing these primates. Radio-collaring of free ranging primates proposes a method of obtaining more accurate and complete data surrounding primate behavior patterns. This in turn can aid in understanding what measures need to be taken to promote the conservation of these species. Radio collaring not only allows for the identification of individuals within a species, increased sample size, more detailed dispersal and range patterns, but also facilitates educational programs which raise awareness for the current biodiversity crisis.